This invention relates to the production of metal such as aluminum from metal chloride dissolved in molten halide solvent bath by electrolyzing the bath in a monopolar or bipolar cell. More particularly, the invention relates to carbonaceous or graphite electrodes used in such cells and to selective use thereof with respect to their wetting or non-wetting characteristics so as to prolong useful electrode life in such cells.
One type of electrolytic cell used in the production of metal, such as aluminum, from metal chloride dissolved in a solvent salt bath includes a terminal anode, at least one intermediate bipolar electrode and a terminal cathode. These electrodes are typically situated in relatively closely spaced, generally parallel relationship wherein opposed anode-cathode faces provide interelectrode spaces through which the molten bath can move and be electrolyzed by passage of current from anode to cathode. Electrolysis of the metal chloride occurring within the interelectrode space results in molten metal depositing at the cathode and chlorine gas collecting at the anode. Cells of this type are described in U.S. Pat. Nos. 3,755,099 and 3,822,195, incorporated herein by reference. One of the important features of these cells is that the anode-to-cathode space or distance should be carefully maintained at a preselected level in order to achieve the high current efficiency and lower power consumption capabilities of the bipolar chloride electrolysis process. Obviously, any amount of wear occurring on either the anode or the cathode surface, as by erosion or other removal of electrode material tends to increase the distance and, accordingly, increase the electrical resistance across the distance between anode and cathode. For the most part, the anode presents little problem since under most conditions chlorine is relatively non-corrosive to the carbonaceous materials employed for electrodes. However, experience has shown that some amount of electrode wear does occur on the cathode surface, and considerable effort has been expended to reducing or relieving this wear condition. Excessive cathode surface wear is a problem, not only in increasing power consumption, as just explained, but can increase the resistance so much that the cell is considered uneconomical to operate, thus necessitating a costly shutdown, repair or replacement of the electrodes, and restarting the cell. In addition to the electrical resistance problems resulting from cathode wear, the carbonaceous material removed from the cathode surface can contaminate the bath. This alone can reach such an extreme as to necessitate shutting down the cell.